This invention relates to an alarm circuit for indicating the presence of abnormal conditions and more particularly to an electronic alarm switch incorporating fail-safe concepts.
Power plant control systems manufacturers and the transportation industry are constantly on the lookout for fail-safe alarm circuits that will guarantee an interlock shutdown when the sensed analog control signal has either disappeared or weakened so that timely action can be taken to place back-up systems on-line or to activate other safeguards to meet the indicated contingency.
Several methods are known in the art to provide fail-safe monitoring of process variable signals. U.S. Pat. No. 3,588,857 discloses means for level detecting a difference signal obtained by differencing two redundant but separate input signals and applying the result to control an oscillator type alarm circuit.
U.S. Pat. No. 3,854,089 discloses a way to employ an LED annunciator to monitor the currents of one or more current sources and an opto-isolator to isolate the voltages being monitored from affecting the operating-voltage of the fail-safe circuitry.
However, most fail-safe circuitry in the art is relatively complex with the use of many active elements, in contrast to this invention which uses only three. Furthermore, this invention details a unique way to combine the virtues of electrically isolating the monitoring circuit from the process variable (PV) signal by means of an opto-isolator element which simultaneously provides direct control over the oscillator controlled alarm.